Low-density alloy and fabrication method thereof

ABSTRACT

A low-density alloy and the fabrication method thereof are disclosed. The alloy comprises, in weight percent, equal to or greater than 15 wt. % but lower than or equal to 22.5 wt. % manganese, equal to or greater than 7.2 wt. % but lower than or equal to 9.0 wt. % aluminum, equal to or greater than 5.1 wt. % but lower than or equal to 7.8 wt. % chromium, equal to or greater than 0.6 wt. % but lower than or equal to 1.2 wt. % carbon and the balance of iron. The golf-club head made from the abovementioned alloy can obtain superior elongation, strength, damping capacity, and corrosion resistance even without any hot/cold working process, such as forging, rolling, etc.; therefore, the fabrication cost thereof can be obviously reduced.

1. FIELD OF THE INVENTION

This invention relates to a low-density alloy, especially to alow-density alloy for making the golf-club head with superiorelongation, strength, damping capacity and corrosion resistancegenerated without any plastic hot/cold working process, such as forging,rolling, etc., and fabrication thereof.

2. BACKGROUND OF THE RELATED ART

To provide better ball-hitting feeling for the golfer, and to enable thegolfer to hit the ball farther and more stably (i.e. longerball-contacting time, higher ball-controlling ability, and lowervibration), many commercial materials have been applied to golf-clubheads, such as 8620 steel, 304 austenitic stainless steel, 17-4PHprecipitation-hardening stainless steel, AISI431/AISI455 high-strengthmartensitic stainless steel, 18Ni(200) maraging steel, Ti-6Al-4V alloy,and SP-700 titanium alloy. Some materials have superior ductility butinsufficient strength. (For example, 8620 steel and 304 austeniticstainless steel have elongations of about 30% but have, tensilestrengths of only about 60 ksi.) Some have very high strength but lowductility. (For example, AISI431/AISI455 high-strength martensiticstainless steel and 18Ni(200) maraging steel have tensile strengths ashigh as 150˜200 ksi but have elongations of only 10% or below.)

For the last decades, many specialists and scholars have developed aseries of Fe—Mn—Al—C-based high-strength and high-ductility alloys, andthe properties of those Fe—Mn—Al—C-based alloys are clearly described inthe following papers.

-   1. G. L. Kayak, “Fe—Mn—Al Precipitation-Hardening Austenitic    Alloys”, Metal Science and Heat Treatment, Vol. 2, 1969, P.95-   2. M. F. Alekseenko, et al., “Phase Composition Structure and    Properties of Low-Density Steel 9G28Yu9MVB”, Metal Science and Heat    Treatment, Vol. 14, 1972, P.187-   3. G. S. Krivonogov, et al., “Phase Transformation Kinetics in Steel    9G28Yu9MVB”, Phys. Met.& Metallog, Vol. 4, 1975, P.86-   4. L. I. Lysak, et al., “Structural and Phase Change in Steel    9G28Yu9MVB During Aging”, Metallogizika, Vol. 59, 1975, P.29-   5. Charles, et al., “New Cryogenic Materials: Fe—Mn—Al Alloys”,    Metal Progress, May, 1981, P.71-   6. C. J. Altstetter, et al., “Processing and Properties of Fe—Mn—Al    Alloys”, Materials Science and Engineering, Vol. 82, 1986, P.13-   7. K. H. Ham, et al., “The Evidence of Modulated Structure in    Fe—Mn—Al—C Austenitic Alloys”, Scripta Metal., Vol. 20, 1986, P.33-   8. P. J. James, “Precipitation of the Carbide (Fe, Mn)₃AlC in an    Fe—Al Alloy”, J. Iron & Steel Inst., January 1969, P.54

From surveying the abovementioned papers, it is found that after thedeformation processes, such as forging and rolling, a solid-solutionheat treatment at the temperature of 950˜1200° C. followed by a rapidquenching, and then an aging heat treatment at the temperature of450˜750° C., the Fe—(28˜35) wt. % Mn—(4.9˜11) wt. % Al— (0.5˜2.0) wt. %C-based alloy becomes a high-strength and high-ductility alloy having anaustenitic-matrix structure, a density of 6.6˜6.8 g/cm³, a tensilestrength of 100˜180 ksi, a yield strength of 90˜160 ksi, and anelongation of 25˜65%.

In order to improve the corrosion resistance, 2.98˜6 wt. % of Cr and0.9˜1.03 wt. % of Mo may be further added into the abovementionedFe—Mn—Al—C-based alloy; the corrosion resistance thereof has beendiscussed in detail in the following papers.

-   1. Jeng-Gong Duh, et al., “Diffusion-Related Kinetics in the    Oxidation-Induced Phase Transformation of Fe-9Al-3Cr-31Mn    Alloys”, J. Electronchem. Soc. Vol. 136, No. 3, March 1989-   2. Jeng-Gong Duh, et al., “Microstructural development in the    oxidation-induced phase transformation of Fe—Al—Cr—Mn—C alloys”,    JOURNAL OF MATERIALS SCIENCE, Vol. 23, 1989-   3. J. G. Duh, et al., “Nitriding behavior in Fe—Al—Mn—Cr—C alloys at    1000-1100° C.”, JOURNAL OF MATERIALS SCIENCE, Vol. 28, 1993-   4. S. C. Chang, et al., “Environment-Assisted Cracking of Fe-32%    Mn-9% Al Alloys in 3.5% Sodium Chloride Solution”, J. CORROSION,    Vol. 51, 1995-   5. J. G. Duh, et al., “Nitriding Kinetics of Fe—Al—Mn—Cr—C alloys at    1000° C.”, JOURNAL OF MATERIALS SCIENCE, Vol. 25, 1990-   6. J. G. Duh, et al.; “High temperature oxidation of    Fe-31Mn-9Al-xCr-0.87C alloys (x=0, 3 and 6)”, JOURNAL OF MATERIALS    SCIENCE, Vol. 25, 1990

Through the developments described above, the Fe—Mn—Al—C-based alloyshave been applied to golf-club heads. The compositions, heat-treatmentand plastic working conditions of the disclosed Fe—Mn—Al—C-based alloysfor the use of making golf-club heads in the prior arts are shown inTable 1 for comparison.

TABLE 1 Application Composition (*: represents an optional additive)Pub. No. Fe Mn Al Cr C Si Mo Cu Nb TW178648 Bal. 22~36 6~8 1.5~2.01.0~1.5 TW185568 Bal. 26~28 6.5~8   5~6 0.9~1.1 0.2~1.5 1.0~1.2 0.9~1.10.02~0.04 US20030077479 Bal. 25~31 6.3~7.8 5.5~9   0.65~0.85 *0.8~1.5 *0.5~1.0  US20030082067 Bal.   28~31.5 7.8~10  *5~7  0.9~1.1 *0.8~1.5 US20050006007 Bal. 25~31  7~10 5~7 0.9~1.1 *0.8~1.5  *0.5~1.0  TW1235677Bal. 23~30 6.3~10  5~9  0.8~1.05 *0.6~1.0  Composition (*: represents anApplication optional additive) Forging and heat treatment Pub. No. Ti CoN condition TW178648 Solid solution at 1030~1050° C. for 1~2 hours andheat treatment at 400~550° C. for 1~2 hours TW185568 Homogenization heattreatment US20030077479 *2~5 Hot forging at 850~1050° C. and heattreatment at 980~1080° C. for 1~24 hours US20030082067 0.35~2.5 Hotforging at 900~1100° C. and heat treatment at 950~1270° C. for 1~24hours US20050006007 Hot forging at 850~1050° C. and hot treatment at980~1080° C. for 1~4 hours and at 500~650° C. for 4~8 hours TW12356770.2~10 *0.2~0.4 Hot forging at 1000~1050° C., and heat treatment at1030~1080° C. for 15~60 minutes and heat treatment at 450~850° C. for4~24 hoursTaiwan patent of Publication No. 178648 (denoted TW178648) disclosed thecompositions of an alloy without chromium, so the alloy has a poorcorrosion resistance. The patent TW185568 disclosed an alloy having abetter corrosion resistance than that disclosed in TW178648, but thegolf-club head made from the alloy still fails the exam of salt-sprayingtest with 5% sodium chloride solution for 48 hours after the golf-clubhead has been polished to burnish.

Except for those discussed above, although the golf-club heads made fromthe alloys disclosed in US patents of Publication No. 20030077479,20030082067 and 20050006007 and Taiwan patent of Publication No. 1235677pass the exams of salt-spraying test with 5% sodium chloride solutionfor 48 hours, but it deserves to mention that the alloys disclosed inabove patents have to be treated by a plastic working process such asforging, rolling and hot/cold working to form a superior surfacecondition and a microstructure of fine and homogeneous poly-crystal froma inferior dendrite structure of cast, so that the forged or workedgolf-club heads are capable of passing the exams of salt-spraying test.

But, a forging process of a forged golf-club head includes several roughand precise forging procedures, which requires many expensive forgingmolds used in the process, so the manufacturing cost is relatively high.Moreover, it limits the shape of a golf-club head very much due to thelimitation of the shape in the forging process, so that the forged orworked golf-club heads do not have varieties, functionalities andartistic shapes like the golf-club heads made by the precision castingprocess. However, when the alloys disclosed in aforementioned patentsmake the golf-club heads by a precision casting process rather than aforging process, the casting-type golf-club head fails in the exam ofthe salt-spraying test eventually.

SUMMARY OF THE INVENTION

For solving the above mentioned problem to enable the alloy to pass theexam of the salt-spraying test in the not only forged condition but alsothe casting condition, this invention provides a low-density alloy,which lowers down the cost of fabricating a golf-club head of theFe—Mn—Al—C-based alloy, and increases the diversity of designing thegolf-club head shape, so the low-density alloy is particularly suitablefor the use of making the golf-club head.

An object of this invention is to provide a low-density alloy and thefabrication method thereof, where the low-density alloy has excellentelongation, strength, damping capacity, and corrosion resistance withoutany plastic hot/cold working, such as forging, rolling, etc.

Another object of this invention is to provide a low-density alloy andthe fabrication method thereof, where the low-density alloy can pass theexam of the salt-spraying test without any plastic hot/cold working,such as forging, rolling, etc., to reduce the cost of production.Additionally, the design flexibility for the golf-club head of theFe—Mn—Al—C-based alloy no longer is restricted by the forging or workingprocess.

Another object of this invention is to provide a low-density alloy andthe fabrication method thereof, where the low-density alloy has a lowdensity ranging from 6.6 to 6.9 g/cm³, an elongation ranging from 30 to77%, a high tensile strength ranging from 100 to 140 ksi, high dampingcapacity and superior corrosion resistance without any plastic hot/coldworking, such forging, rolling, etc.

For achieving the abovementioned objects, a low-density alloy accordingto an embodiment of this invention comprises, by weight percent (wt. %),greater than or equal to 15 wt. % but lower than or equal to 22.5 wt. %manganese, larger than or equal to 7.2 wt. % but lower than or equal to9.0 wt. % aluminum, larger than or equal to 5.1 wt. % but lower than orequal to 7.8 wt. % chromium, larger than or equal to 0.6 wt. % but lowerthan or equal to 1.2 wt. % carbon and is the balance of iron.

For achieving the abovementioned objects, a fabrication method of alow-density alloy, according to an embodiment of this invention,includes melting a raw material, which comprises, by weight percent (wt.%), greater than or equal to 15 wt. % but lower than or equal to 22.5wt. % manganese, larger than or equal to 7.2 wt. % but lower than orequal to 9.0 wt. % aluminum, larger than or equal to 5.1 wt. % but lowerthan or equal to 7.8 wt. % chromium, larger than or equal to 0.6 wt. %but lower than or equal to 1.2 wt. % carbon and the balance of iron, toform an alloy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the depth profile of the alloying elementson the surface layer of the Fe-32Mn-8.2Al-5.1Cr-0.9C alloy, by weightpercent (wt. %), for comparison with that of this invention.

FIG. 2 is a diagram showing the depth profile of the alloying elementson the surface layer of the Fe-28Mn-8.2Al-5.1Cr-0.9C alloy, by weightpercent (wt. %), for comparison with that of this invention.

FIG. 3 is a diagram showing the depth profile of the alloying elementson the surface layer of the Fe-19.5Mn-8.2Al-5.1Cr-0.9C alloy, by weightpercent (wt. %), according to an embodiment of this invention.

FIG. 4 is a diagram showing the potentiodynamic polarization curvesperformed in 5% NaCl solution for both alloys ofFe-28Mn-8.2Al-5.11Cr-0.9C and Fe-19.5Mn-8.2Al-5.1Cr-0.9C, by weightpercent, containing different Mn contents respectively.

DETAILED DESCRIPTION OF TIRE INVENTION

The alloy of this invention is based on iron (Fe), manganese (Mn),aluminum (Al), chromium (Cr) and carbon (C)) where Mn is one ofstabilized elements for austenite phase, and the greater Mn content ofan alloy makes more austenite phase. The austenite has more slip systemsdue to the FCC structure to enhance the elongation, so to add a moderateamount of Mn to the alloy is beneficial to enhance the elongation of thealloy. However, according to the research of the inventors, Mn elementin the alloy enhances the elongation but deteriorates the corrosionresistance, since Mn is oxidized with ease in atmospheric environment,and the manganese oxide has poor adherence to the substrate and tends tospall off, so that it cannot provide the protection to prevent oxidationfrom proceeding to the interior of the substrate. Further, analyzingoxides of the passivated layer on the Fe—Mn—Al—Cr—C alloy surface byX-Ray Photoelectron Spectroscopy/Electron Spectroscopy of ChemicalAnalysis (XPS/ESCA) finds that the oxides of the passivated layerconsists primarily of anti-corrosion Cr₂O₃ and Al₂O₃ oxides, andnon-corrosion-resistant oxides of manganese oxide MnO(Mn₃O₄), Mn₂O₃ andferric oxide FeO(Fe₃O₄), Fe₂O₃. Based on the analysis of the passivatedlayer, a greater Mn content in the alloy results in a higher proportionof manganese oxides on the passivated layer. As a result, thenon-continuous passivated protection layer with anti-corrosion Cr₂O₃ andAl₂O₃ oxides is formed on the surface of the alloy containing a higherMn content. In corrosive environment, the corrosion attack is observedto be took place in the region devoid of the passivated protection layeror the region of the manganese oxides, and then extends to the interiorof the substrate to form the localized pits of the pitting corrosionrather than the general corrosion. This is in agreement with thecorroded surface result of the Fe—Mn—Al—Cr—C alloy after thesalt-spraying test.

Therefore, in order to improve the pitting corrosion resistance of theFe—Mn—Al—Cr—C alloy in the condition without any hot/cold workingprocess, such as forging, rolling, etc., and aim at passing the exam ofsalt-spraying test with 5% sodium chloride solution for 48 hours for agolf-club head made from the Fe—Mn—Al—Cr—C alloy, the inventors carriedout an experiment first to investigate the influence of the decrease inthe Mn content of the alloy on the oxide constitution of the passivatedlayer on the surface of the alloy. FIGS. 1 through 3 represent the depthprofiles of the oxides on the surface of theFe—(32,28,19.5)Mn-8.2Al-5.1Cr-0.9C alloys (in weight percent, wt. %)having 32, 28 and 19.5 wt. % Mn, respectively. From those drawings, itis found that the ratio of the manganese oxides in the passivated layerdecreases with decreasing the Mn content of the alloy. As decreasing theMn content to 19.5 wt. %, the ratio of the manganese oxides has a greatdecrease. On the contrary, aluminum oxide and chromium oxide have anobvious increase in ratio. FIG. 4 shows potentiodynamic polarizationcurves of both the Fe-28Mn-8.2Al-5.1Cr-0.9C andFe-19.5Mn-8.2Al-5.1Cr-0.9C alloys (in weight percent) performed in 5%NaCl solution to illustrate the corrosion resistance. From FIG. 4, it isfound that when the Mn content was lowered to 19.5 wt. %, there is agreat decrease in the passivated current intensity (I_(p)), and anobvious increase in the passivated potential (ΔE) and the pittingpotential (E_(pp)). Accordingly, the alloy has a much better corrosionresistance than another alloy of Fe-28Mn-8.2Al-5.1Cr-0.9C with 28 wt. %Mn, so that the Fe-19.5Mn-8.2Al-5.1Cr-0.9C alloy can pass the exam ofsalt-spraying test with 5% NaCl for 48 hours without any hot/coldworking, such as forging, rolling, etc. However, as abovementioned, theMn content is raised to stabilize the austenite phase, so as to increasethe elongation. Thus when the Mn content is less than 15 wt. %, theelongation of the alloy is not satisfied. On the contrary, as addingmore than 22.5 wt. % Mn, there is an obvious deterioration in thecorrosion resistance of the alloy. Therefore, in order to obtain both ahigh corrosion resistance and an excellent toughness, the Mn content ofthe alloy should be controlled to be equal to or greater than 15 wt. %but lower than or equal to 22.5 wt. %.

Aluminum (Al) is one of the stabilized elements for the ferrite phase,so a higher Al content of an alloy makes more ferrite phase and lessaustenite phase to decrease the elongation of the alloy. Further, anexcess of Al in the alloy tends to form the very brittle D0₃ orderedphase to severely destroy the elongation of the alloy; meanwhile, Al isalso one of the constituent elements of the (Fe,Mn)₃AlC_(x) carbideacting as precipitation-hardening to strengthen the Fe—Mn—Al—C-basedalloys. Once Al content is not high enough, the strength of the alloy isinsufficient due to the lack of the (Fe,Mn)₃AlC, carbides. Besides,according to the research of the inventors, insufficient Al proportionsuppresses anti-corrosion oxide of Al₂O₃ to be formed on the surface toreduce the corrosion resistance of the alloy. Therefore, in order toobtain a high corrosion resistance, a high strength and an excellentelongation for the alloy with Mn content between 15 to 22.5 wt. %, theAl content of the alloy should be controlled to be equal to or greaterthan 7.2 wt. % but lower than or equal to 9.0 wt. %. Chromium (Cr), arather active element, has a strong tendency to form a protective layerof chromium oxide Cr₂O₃ on the surface of the alloy to enhance thecorrosion resistance of the alloy. However, Cr is an element not only ofthe ferrite-former but also of the carbide-former. Insufficient Crcontent cannot provide an enough corrosion resistance for an alloy. But,excess Cr may not only reduce the elongation but also deteriorate thecorrosion resistance of the alloy attributed to the formation ofchromium carbide Cr₇C₃ along the grain boundaries. Since the formationof the Cr₇C₃ carbide depletes its surrounding Cr, along with theprecipitation of the carbide, the surrounding region is lacked in Cr andthen the alloy becomes much sensitive to the intergranular corrosion.Therefore, in order to obtain a high corrosion resistance and inhibitthe formation of the Cr₇C₃ carbide for the alloy with Mn content between15 to 22.5 wt. %, the Cr content of the alloy should be controlled to beequal to or greater than 5.1 wt. % but lower than or equal to 7.8 wt. %,and the C content should be controlled to be equal to or greater than0.6 wt. % but lower than or equal to 1.2 wt. %. Additionally, to add alittle amount of silicon (Si) and molybdenum (Mo) properly increases thecorrosion resistance also.

A low-density alloy, according to this invention, comprises, in weightpercent, 15˜22.5 wt. % Mn, 7.2˜9.0 wt. % Al, 5.1˜7.8 wt. % Cr, 0.6˜1.2wt. % C and the balance of Fe, and the addition of 0˜1.5 wt. % Mo canincrease the pitting corrosion resistance. Further, in anotherembodiment, to add 0.7 wt. % Si at most improves the fluidity of thepresent alloy in molten state. An alloy, according to this invention,has a low density ranging from 6.6 to 6.9 g/cm³, an excellent elongationranging from 30 to 77%, a tensile strength ranging from 100 to 140 ksi,a high damping capacity and a high corrosion resistance, and succeeds inthe exam of the salt-spraying test without any plastic hot/cold workingprocess to reduce the fabrication cost.

A fabrication method, according to this invention, is to melt rawmaterials, which include larger than or equal to 15 wt. % but lower thanor equal to 22.5 wt. % Mn, larger than or equal to 7.2 wt. % but lowerthan or equal to 9.0 wt. % Al, larger than or equal to 5.1 wt. % butlower than or equal to 7.8 wt. % Cr, larger than or equal to 0.6 wt. %but lower than or equal to 1.2 wt. % C, 1.5 wt. % Mo at most, 0.7 wt. %Si at most and the balance of Fe, where the melting process can be anatmospheric melting process, a vacuum melting process, or a reducingatmospheric melting process. The molten alloy is then poured into themolds to form the casts. Without any plastic hot/cold working process,the casts are followed by the processes of sand-blasting, grinding,welding, drilling, polishing, surface treating, artistic working, etc.to produce the casting-type golf-club heads. The producing processesfurther optionally include a solid-solution heat treatment at 950˜1200°C. for 0.5˜10 hours and then an aging heat treatment at 500˜700° C. for0˜10 hours to further increase the ductility and strength of the alloy.Since the alloy, according to this invention, has an excellent toughnesseven in the cast state, the alloy is also suitable to be subject to aseries of plastic hot/cold working processes to produce the forging-typegolf-club heads or the complex ones of the combination of casting andforging-types.

For example, an alloy, according to a better embodiment of thisinvention, comprises 22.1 wt. % Mn, 8.01 wt. % Al, 6.21 wt. % Cr, 0.99wt. % C and the balance of Fe. The alloy may be melted with ahigh-frequency induction furnace, and then, the molten alloy is pouredinto pre-heated dewaxed shell molds of golf-club heads. After the castswith shell molds cool down, the casts are processed by shell removing,sprue-gate cutting, sand-blasting, grinding, welding, drilling,polishing, surface treating, artistic working, etc. to produce thecasting-type golf-club heads. Even without any plastic hot/cold workingprocess, the golf-club head not only is capable of passing the exam ofthe salt-spraying test With 5% NaCl solution for 48 hours but alsopossesses a density as low as 6.74 g/cm³, an elongation as high as38.6%, and a tensile strength as great as 112.1 ksi. Therefore, thefabrication cost of golf-club heads can be greatly reduced.

An alloy, according to a better embodiment of this invention, comprises16.3 wt. % Mn, 8.56 wt. % Al, 5.16 wt. % Cr, 1.10 wt. % C and thebalance of Fe. In addition to the manufacturing process described in thefirst embodiment, the golf-club head castings are further subject tosolid-solution heat treatment at 1050° C. under vacuum for 1 hour toenhance their mechanical properties. Accordingly, the golf-club head notonly is capable of passing the exam of the salt-spraying test with 5%NaCl solution for 48 hours but also possesses a density as low as 6.69g/cm³, an elongation as high as 76.9%, and a tensile strength as greatas 118.7 ksi even though it is free of any plastic hot/cold workingprocess. Therefore, the fabrication cost of golf-club heads can begreatly reduced.

An alloy, according to yet another better embodiment of this invention,comprises 19.2 wt. % Mn, 7.78 wt. % Al, 6.73 wt. % Cr, 1.03 wt. % C,0.21 wt. % Si and the balance of Fe. In addition to the manufacturingprocess described in the first embodiment, the golf-club head castingsare further subject to solid-solution heat treatment at 1050° C. undervacuum for 1 hour to enhance their mechanical properties. Accordingly,the golf-club head not only is capable of passing the exam of thesalt-spraying test with 5% NaCl solution for 48 hours but also possessesa density as low as 6.78 g/cm³, an elongation as high as 66.9%, and atensile strength as great as 115.3 ksi even though it is free of anyplastic hot/cold working process. Therefore, the fabrication cost ofgolf-club heads can be greatly reduced.

According to the abovementioned illustration, utilizing the fabricationmethod, according to embodiments of this invention, forms an alloy withlow density ranging from 6.6 to 6.9 g/cm³ for use of making thegolf-club heads without any plastic hot/cold working, such as forging,rolling, etc., and then the alloy has the excellent ductility rangingfrom 30 to 77%, high tensile strength between 100 to 140 ksi, highdamping capacity and high corrosion resistance (which can pass the examof salt-spraying test with 5% NaCl for 48 hours for the golf-club head).The proper design for the alloy improves the fluidity in its moltenstate, casting ability and plastic working ability to reduce the costand time of making the golf-club head, and increases also the designcapacity of a golf-club head shape, as compared to that made from theconventional Fe—Mn—Al—C-based alloys requiring plastic working processto enhance their corrosion resistance. So, the alloys are suitable forthe golf-club heads.

In conclusion, an golf-club bead made from an alloy, whose compositionsand the fabrication method are disclosed in this invention, has lowdensity, high strength, high toughness, high damping capacity and highcorrosion resistance, and it can pass the exam of salt-spraying testwith 5% NaCl for 48 hours without any plastic deformation process;moreover, due to increasing the fluidity, it is easy to cast the shapewith small words, trenches of the striking face and thin part of thehead without mechanical carving, so that the fabrication cost and defectrate of the products are lowered down very much.

Those embodiments described above are to clarify the present inventionto enable the persons skilled in the art to understand and use thepresent invention; however, the embodiments are not intended to limitthe scope of the present invention; therefore, any equivalentmodification and variation according to the spirit of the presentinvention is still to be included within the scope of the claims of thepresent invention stated below.

1. A low-density alloy comprising greater than or equal to 15 wt. % butlower than or equal to 22.5 wt. % manganese, greater than or equal to7.2 wt. % but lower than or equal to 9.0 wt. % aluminum, greater than orequal to 5.1 wt. % but lower than or equal to 7.8 wt. % chromium,greater than or equal to 0.6 wt. % but lower than or equal to 1.2 wt. %carbon and the balance of iron.
 2. A low-density alloy according toclaim 1, further comprising 1.5 wt. % molybdenum at most.
 3. Alow-density alloy according to claim 1, further comprising 0.7 wt. %silicon at most.
 4. A low-density alloy according to claim 2, furthercomprising 0.7 wt. % silicon at most.
 5. A low-density alloy accordingto claim 1, having a density in a range from 6.6 to 6.9 g/cm³, anelongation in a range from 30 to 77% and a tensile strength in a rangefrom 100 to 140 ksi.
 6. A fabrication method of a low-density alloy,including melting a raw material, which comprises greater than or equalto 15 wt. % but lower than or equal to 22.5 wt. % manganese, greaterthan or equal to 7.2 wt. % but lower than or equal to 9.0 wt. %aluminum, greater than or equal to 5.1 wt. % but lower than or equal to7.8 wt. % chromium, greater than or equal to 0.6 wt. % but lower than orequal to 1.2 wt. % carbon and the balance of iron, to form an alloy. 7.A fabrication method of a low-density alloy according to claim 6,wherein said raw material further comprises 1.5 wt. % molybdenum atmost.
 8. A fabrication method of a low-density alloy according to claim7, wherein said raw material further comprises 0.7 wt. % silicon atmost.
 9. A fabrication method of a low-density alloy according to claim8, further including a step of heat-treating said alloy at a temperaturein a range from 950 to 1200° C. for a duration in an interval from 0.5to 10 hours, and then at a temperature in a range from 500 to 700° C.for a duration up to a limit of 10 hours.
 10. A fabrication method of alow-density alloy according to claim 7, further including a step ofheat-treating said alloy at a temperature in a range from 950 to 1200°C. for a duration in an interval from 0.5 to 10 hours, and then at atemperature in a range from 500 to 700° C. for a duration up to a limitof 10 hours.
 11. A fabrication method of a low-density alloy accordingto claim 6, wherein said raw material further comprises 0.7 wt. %silicon at most.
 12. A fabrication method of a low-density alloyaccording to claim 11, further including a step of heat treating saidalloy at a temperature in a range from 950 to 1200° C. for a duration inan interval from 0.5 to 10 hours, and then at a temperature in a rangefrom 500 to 700° C. for a duration up to a limit of 10 hours.
 13. Afabrication method of a low-density alloy according to claim 6, furtherincluding a step of heat-treating said alloy at a temperature in a rangefrom 950 to 1200° C. for a duration in an interval from 0.5 to 10 hours,and then at a temperature in a range from 500 to 700° C. for a durationup to a limit of 10 hours.
 14. A fabrication method of a low-densityalloy according to claim 6, wherein said step of melting said rawmaterial is an atmosphere melting, a vacuum melting, or a reducingatmosphere melting.
 15. A fabrication method of a low-density alloyaccording to claim 6, wherein said alloy has a density in a range from6.6 to 6.9 g/cm³, an elongation in a range from 30% to 77% and a tensilestrength in a range from 100 ksi to 140 ksi.